High fluctuations in the nickel price have increased the interest to low-nickel and nickel-free alternatives of Cr—Ni-alloyed austenitic stainless steels. When describing the element content in the following, the content is in weight %, if not otherwise mentioned. Manganese-alloyed 200-series austenitic stainless steels have generally equal formability compared to Cr—Ni-alloyed 300-series grades, and also their other properties are comparable. However, most manganese-alloyed grades, especially those with particularly low nickel content from 0% to 5%, are susceptible to delayed cracking phenomenon, which prevents their use in applications where severe deep-drawing operations are needed. Another drawback of the low-nickel grades currently available is that they have reduced the chromium content in order to ensure fully austenitic crystal structure. For instance, low-nickel grades with around 1% nickel contain typically only 15% chromium, which impairs their corrosion resistance.
One example of a low-Ni Mn-alloyed steel grade is grade AISI 204 (UNS S20400) that can be made as a modified version by alloying with copper, Cu. The new copper alloyed material in the standard is named as S20431 according to the standard ASTM A 240-09b and EN specified grade 1.4597. These steels are widely used for domestic appliances, shallow pots and pans and other consumer products. However, the currently available steels are very susceptible to delayed cracking, and therefore cannot be used in applications where material is subjected to deep drawing.
Some austenitic stainless steel grades with reduced nickel content designed to be resistant to delayed cracking have been proposed. GB patent 1419736 discloses an unstable austenitic stainless steel with low susceptibility to delayed cracking, which is based on low contents of C and N. However, the steel in question has minimum Ni content specified as 6.5%, impairing the cost-efficiency of the steel.
WO publication 95/06142 discloses an austenitic stainless steel, which is made resistant to delayed cracking by limiting the C and N content and by controlling the Md30-temperature describing the austenite stability of the steel. However, the steel of this WO publication contains at the minimum 6% nickel, and is thus not cost efficient.
EP patent 2025770 discloses a nickel-reduced austenitic stainless steel, which is made resistant to delayed cracking by controlling the Md30-temperature. However, the steel of this EP patent contains at the minimum 3% nickel, reducing the cost-efficiency of the steel.
In addition, numerous alloys have been proposed to find cost efficient alternatives for conventional Cr—Ni alloyed steel grades. However, none of the existing alloys combine low nickel content (about 1%) and high resistance to delayed cracking.
For instance, EP patent 0694626 discloses an austenitic stainless steel containing 1.5-3.5% nickel. The steel contains 9-11% manganese, which however may impair the surface quality and corrosion resistance of the steel. U.S. Pat. No. 6,274,084 discloses an austenitic stainless steel with 1-4% nickel. U.S. Pat. No. 3,893,850 discloses a nickel-free austenitic stainless steel containing at the minimum 8.06% manganese and no more than 0.14% nitrogen. EP patent 0593158 discloses an austenitic stainless steel containing at least 2.5% nickel, thus not exhibiting optimum cost-efficiency. Furthermore, none of the above-mentioned steels has been designed to be resistant to delayed cracking, which limits their use in such applications where severe forming operations need to be carried out.